Cal Fire says the Wragg Fire, burning near Lake Berryessa, flared up Tuesday afternoon.
Mandatory evacuations were ordered for Sky Ranch, Mix Canyon Road and Gates Canyon Road. More than 130 homes are threatened.
An evacuation center has been set up at the Ulatis Community Center at 1000 Ulatis Drive in Vacaville.
The flare-up was said to be burning 400 acres of brush at the bottom of a canyon, outside of the containment lines, and terrain is difficult for firefighters.
Since starting almost a week ago, the Wragg Fire has burned more than 6,991 acres in Napa and Solano counties and is about 80 percent contained.
The fire has destroyed two outbuildings and damaged three outbuildings and one home.